This invention relates to differential pulse code modulation (DPCM) of NTSC color composite TV signals, and more particularly, to predictors used in the DPCM process.
In any digital encoding system, it is important that the coded signal should represent an analogue signal from which it is derived as closely as possible with a minimum of redundancy, so as to economize on the bandwidth of any channel over which the signal is to be transmitted. One of the most efficient methods of transmission is a process known as Differential Pulse Code Modulation (DPCM) in which, instead of digitizing the instantaneous value of the signal, the difference between the instantaneous signal value and a previous value of the signal is digitized. DPCM is generally classified as a "predictive" type of encoding in that it exploits the predictability, i.e., redundancy, of a signal to achieve a reduced digit rate for pulse code modulation (PCM) transmission.
Shown in FIG. 1 is a block diagram of a typical DPCM system having a transmitter 1 and receiver 11. As shown therein, the analog information signal at terminal 2 is sampled in analog-to-digital convertor 3 controlled by sampling pulses from timing control 5. The current digitized sample is supplied to a subtractor 7 where it is compared to a predicted value of that sample and the difference between the actual and predicted values is quantized in quantizer 9. The quantized difference signal is fed to an adder 13 where the current sample is then reconstructed by adding the difference signal and predicted value. The reconstructed sample is then fed to shift registers SR1-SRN having different lengths and is advanced through the shift registers under control of the same sampling pulses as are fed to A/D converter 3 from timing control device 5. Since the shift registers are of unequal lengths, N different reconstructed samples are present at their output terminals at any given time. These N reconstructed samples are multiplied by weighting co-efficients .alpha..sub.1 -.alpha..sub.N and are combined in adder 15 to obtain the predicted value for the current sample. The quantized difference signal from the quantizer 19 is also sent to the receiver 11 where it is combined in adder 17 with the predicted value of that sample. The reconstructed sample value is then supplied simultaneously to digital-to-analog converter 4 and shift registers SR21-SR2N. Digital-to-analog converter 4 reconverts the video signal to its original analog form and shift registers SR21-SR2N store the reconstructed samples to be used by weighting means .alpha..sub.21 -.alpha..sub.2N and adder 19 to provide predicted sample values identical to those obtained in the transmitter 1. Shift registers SR21-SR2N and weighting means .alpha..sub.21 -.alpha..sub.2N are identical to their counterparts in the transmitter 1. While FIG. 1 shows the different parts in the receiver to be controlled by timing control device 5, it should be understood that this is for illustrative purposes only, and that in reality the timing control signals for the receiver will be obtained from the transmitter, e.g., they may be derived from the bit rate of the DPCM signals so that the timing control pulses at the receiver are identical in phase and frequency to the timing signals used in the transmitter.
For a monochrome television signal redundancy is high, as is evidenced by its signal power spectrum which has a predominance of energy at low frequencies. Predictability of monochrome signals is high since most television signals contain large areas of constant or near-constant brightness; i.e., given the amplitude of any signal sample there is a high probability that the following sample will have very nearly the same value.
The National Television Standard Committee (NTSC) color television system uses the same luminance signal as in the monochrome system, and transmits the color, or chrominance, in the form of a 3.58 MHz phase-and-amplitude-modulated color subcarrier signal modulated onto the luminance signal. The presence of the color subcarrier imposes considerable difficulties on the design of predictors for DPCM of composite signals since even within regions of uniform luminance the value of the sampled picture elements (pels) varies according to the color subcarrier. One method of predicting the color composite signal is to use as a prediction a previous sample having the same color subcarrier phase as the pel to be predicted. It is advantageous to select this sample from a point in the field as close as possible to the pel to be predicted in order to minimize the probable variance of the luminance signal. This can be more clearly understood by referring to FIG. 2 which illustrates the spatial distribution and phase relationship of color subcarrier samples from adjacent field lines for a sampling rate of 10.7 MHz (3 times the subcarrier frequency). The waveforms in FIG. 2 represent the color subcarrier in adjacent lines of the TV field, and the dots on the waveforms represent the sampling times. As shown therein, the minimum possible distance d.sub.1 between samples of the same line having the same subcarrier phase is the distance corresponding to one period of the subcarrier. In order to obtain this distance between samples, the sampling rate should be an integral multiple of the subcarrier frequency, f.sub.sc. The most common sampling rate is 3f.sub.sc = 10.7 MHz since it is the lowest integral multiple of the subcarrier frequency above the Nyquist rate (8.4 MHz) for the color composite signal. If a sample from a previous line is to be used, the minimum possible distance d.sub.2 between samples having the same subcarrier phase is the vector sum of the distance d.sub.3 between the lines and the distance d.sub.4 corresponding to half a period of the subcarrier. This is due to the fact that the subcarrier frequency is an odd multiple of half the line frequency and, therefore, the subcarrier signals on adjacent field lines are always 180.degree. out of phase. As shown in FIG. 2, a sampling rate of 3f.sub.sc is also compatible with the selection of samples from previous lines.
The disadvantage of this sampling rate is its inefficiency. The bandwidth of the NTSC color composite video is 4.2 MHz, and 3f.sub.sc = 10.7 MHz is far above the required Nyquist rate (8.4 MHz).
One method of sampling at a rate other than 3 times the subcarrier frequency is described in U.S. Pat. No. 3,891,994. According to that method, the color signal from a previous field line is sampled at a rate which is n/m times the subcarrier frequency, where n and m are both small integers. The samples are then fed into a plurality of shift registers having different lengths, so that for any given sampling frequency the proper combination of shift registers outputs can be selected to provide the samples from the previous line having the same subcarrier phase. For a sampling rate of n/m times the subcarrier frequency, a complete cycle of sampling will extend over m subcarrier periods and, since the subcarrier signals on adjacent field lines are 180.degree. out of phase, a co-phased sample from the preceding line can always be found within a distance of .+-.m/2 subcarrier periods. It can be shown that for m = 1 or 2, a co-phased sample from a preceding line can always be found at -1/2 of the subcarrier cycle, thereby obtaining the minimum distance d.sub.2 shown in FIG. 1. However, this limitation on the value of m limits severely the desirable range of sampling frequencies. For a Nyquist rate of 8.4 MHz, there is only one available sampling rate, i.e., (5/2) f.sub.sc, above the Nyquist rate and yet below the inefficient sampling rate of 3f.sub.sc.
There is, therefore, a need for a method of predicting color composite TV signals in which the distance between co-phased samples can always be kept at a minimum, and which provides a greater flexibility in the selection of the sampling rate so that the efficiency of the TV signal transmission system may be maximized.